Certified Cable Matters 3.3ft Black Thunderbolt Cable (Thunderbolt 2 Cable) paired with Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter allows my older (late 2013 d.) iMac as a second monitor for my iMac Pro while retaining the functionality of the old machine. If the old iMac had had a minimum display port instead of Thunderbolt 2, the solution would have been significantly cheaper. Since the cost of both was about $93.